The present invention relates generally to a position sensor, and more particularly to a position sensor provided in an exposure apparatus that transfers a fine circuit pattern. The present invention is suitable, for example, for an exposure apparatus that uses ultraviolet light (“UV”) and extreme ultraviolet (“EUV”) light as an exposure light source and purges an exposure optical path.
A reduction projection exposure apparatus has been conventionally employed which uses a projection optical system to project a circuit pattern formed on a mask or a reticle onto a wafer, etc. to transfer the circuit pattern, in manufacturing such a fine semiconductor device as a semiconductor memory and a logic circuit in photolithography technology.
The minimum critical dimension (“CD”) to be transferred by the projection exposure apparatus or resolution is proportionate to a wavelength of light used for exposure, and inversely proportionate to the numerical aperture (“NA”) of the projection optical system. The shorter the wavelength is, the better the resolution is. Along with recent demands for finer semiconductor devices, a shorter wavelength of ultraviolet light has been promoted from an ultra-high pressure mercury lamp (i-line with a wavelength of approximately 365 nm) to KrF excimer laser (with a wavelength of approximately 248 nm) and ArF excimer laser (with a wavelength of approximately 193 nm). However, the lithography using the ultraviolet light has the limit to satisfy the rapidly promoting fine processing of a semiconductor device, and a reduction projection optical system using EUV light with a wavelength of 10 to 15 nm shorter than that of the ultraviolet (referred to as an “EUV exposure apparatus” hereinafter) has been developed to efficiently transfer a very fine circuit pattern.
The projection optical system is also required to improve throughput as the number of sheets exposed per unit of time. The improved throughput needs the shorter exposure time for each object to be exposed, and the increased exposure light intensity or light quantity or dose to be irradiated onto the object per unit of time. However, the light with a short wavelength is easily subject to absorptions in a material, and its light intensity remarkably decreases when the light transmits in the air or oxygen. Accordingly, the reduction projection optical system that uses light with a short wavelength as exposure light, such as F2 laser and EUV light, closes the space for the optical path area through which the exposure light transmits, and purges the closed space with highly-purity gas (e.g., high-purity purge gas of helium and nitrogen) which is free of impurities, such as organic materials and oxygen, or vacuums up the optical path area through which the exposure light transmits so as to maintain the dose that reaches the wafer.
In particular, the EUV light remarkably decreases its light quantity after passing through a lens, and its light quantity becomes almost zero on a wafer when the EUV light is irradiated on the wafer through an optical system that uses a lens as used for visual light and UV light. The EUV exposure apparatus thus maintains light quantity on the wafer, by closing the space around the exposure light's optical path, by highly vacuuming the space, and by providing an optical system with only mirrors.
The conventional exposure apparatus forms a closed space with a diaphragm between a purged space that purges with purge gas or vacuums the space around a light source, an illumination optical system, a reticle, a projection optical system, and a stage, and an exposure light's optical path, and an external space outside the purged space. The exposure apparatus needs various sensing optical systems, such as an off-axis alignment (“OA”) optical system, a reticle alignment optical system, a focus detecting system, and a wafer position-sensing interferometer.
An OA optical system for detecting an alignment mark on the wafer and thereby a wafer position preferably locates an objective lens closer to the exposure area for a shorter interval or baseline amount between the exposure position and a measurement position of the OA optical system. This is because a wafer is moved to the exposure position by the baseline amount after the OA optical system finishes the alignment, and the alignment accuracy needs a stable and small baseline amount for reduced errors. This means that part of the OA optical system should be located in the purged space.
The reticle alignment optical system for detecting a reticle's position should arrange its part in the purged space since the reticle is located on the exposure light's optical path. In addition, the focus detection system and wafer position-sensing interferometer etc. should arrange their parts in the purged space because their objects to be detected are located in the purged space.
Therefore, these sensing optical systems arranged across the purged space and the external space maintain the closed space and its arrangement with a transmission window member as a diaphragm on the optical path that partitions the purged and external spaces.
The purged space has a pressure different from the external space due to a supply of purge gas or a vacuum atmosphere. A difference between two spaces is particularly very large when the purge space is vacuumed. Thus, a transmission window member as a diaphragm that partitions two spaces receives a large force, and often deforms and/or decenters. These deformation and decentering of the transmission window member on the optical path in the detection system have not been expected in the design, and result in magnification variance, color shift and aberration, such as distortion, deteriorating detection accuracy.
Referring now to FIG. 14, a description will be given of a deformation of the transmission window member caused by a pressure difference. FIG. 14 is a schematic sectional view of the transmission window member deformed by the pressure difference. FIG. 14A shows a transmission window member 1000 at a diaphragm 1100 that partitions a purged space PE and an external space OE. Initially, the transmission window member 1000 does not receive any force or deform.
When the purged space PE is, for example, vacuumed, the pressure in the purged space PE decreases and the transmission window member 1000 receives a force P1 toward the purged space PE, as shown in FIG. 14B, deforming like a meniscus lens. On the other hand, when high-purity purge gas is supplied to the purged area PE to increase its pressure, a force reverse to the force P1 applies to the transmission window member 1000.
Since the transmission window member 1000 perpendicularly receives the force P1, the generated birefringence directs perpendicular to the polarized direction of the incident light and seldom affects the optical performance. However, the diaphragm 1100 that holds the transmission window member 1000 generates a force P2 in response to the force P1 applied to the transmission window member 1000, which force P2 generates birefringence parallel to the polarized direction of the incident light and affects the polarization of the incident light.
When the purged space PE is vacuumed, extremely large force applies to the diaphragm 1100 and the transmission window member 1000, and the diaphragm 1100 conceivably deforms and distorts, as shown in FIG. 14C. FIG. 14C schematically shows the deformed diaphragm 1100 by an angle θ in the purged space PE. Then, the transmission window member 1000 deforms with the diaphragm 1100 by an angle θ due to the deformation under a pressure difference. In other words, the decentering element includes not only the angle θ relative to the optical axis (which is referred to as an “inclined decenter” hereinafter), but also a shift Δd in a direction perpendicular to the optical axis associated with the inclined decenter (which is referred to as a “parallel decenter” hereinafter).
These deformations of the transmission window member possibly result from manufacture errors and changes with time. Regular adjustments need to correct changes with time, and otherwise the measurement accuracy would greatly deteriorate.
On the other hand, it is conceivable to arrange all the elements of the sensing optical system in the purged space instead of arranging part of them in the purged space and the rest in the external space. However, they include a heat source that thermally deforms a holding mechanism and other members, offsets a projection optical system, and deteriorates the measurement accuracy. Therefore, it is not possible to arrange all the elements of the sensing optical system in the purged space.